CH3COOH.TiO2: An excellent catalyst for the green nitration of toluene by N2O4

Abstract

The solvent-free nitration of toluene with N2O4 gas over solid acid catalysts is a green reaction for preparing the mono-nitrotoluene (NT) isomers. The acid-modified catalysts are more efficient than common catalysts for this type of reaction. For this purpose, a titanium dioxide (TiO2) catalyst is synthesized with a sol-gel method and modified by acetic acid to increase catalytic properties. The acid-modified TiO2 (CH3COOH·TiO2) is characterized by different analyses. To optimization of toluene nitration conditions, reaction temperature (X1) (30 < X1 < 60 °C), N2O4/toluene molar ratio (X2) (0.5 < X2 < 2), and the amount of catalyst (X3) (0.05 < X3 < 0.3 g) factors were investigated by Minitab software with the CCD-RSM. Three responses including the selectivity of meta-NT isomer (Sm), the ratio of para-NT to ortho-NT selectivity (Sp/o), and the selectivity of by-products (Sbp) were considered for the optimization. Statistical parameters were applied to evaluate the goodness of fitting for the models. Optimum values for X1, X2, and X3 parameters are 57.9 °C, 1.91, and 0.25 g, respectively. The conversion of toluene under these conditions is 93.2%. The comparison of Sm, Sp/o, and Sbp in CH3COOH.TiO2 (1.51%, 0.8, and 5.48%, respectively) with Fe2O3 (8.5%, 0.43, and 20.57%, respectively), SiO2 (8.43%, 0.48, and 16.24%, respectively), TiO2 (5.9%, 0.57, and 13.87%, respectively), TiO2–Fe2O3 (4.72%, 0.64, and 9.18%, respectively), and TiO2–SiO2 (4.42%, 0.67, and 5.73%, respectively) catalysts show that this catalyst has a low Sm and Sbp as well as a higher Sp/o than other mentioned catalysts. The mechanism of the mentioned reaction is reviewed in the presence of CH3COOH·TiO2 catalyst. The high stability of the CH3COOH·TiO2 is proved by the reusability test, and it is found that its stability against inactivation is more than the TiO2 catalyst.